Stabilizing organo-siloxanes



Patented Nov. 27, 1945 STABILIZING ORGAN O-SILOXANES Rob Roy McGregor, Verona, and Earl Leathen Warrick, Pittsburgh, Pa., assignors to Corning Glass Works, Corning, N. Y.,

New York a corporation of No Drawing. Application June 22, 1944, Serial No. 541,667

'7 Claims.

This invention relates to organo-siloxanes, and particularly to the stabilization thereof.

This application is a continuation-in-part of our copending application Serial Number 432,530, filed February 26, 1942, and assigned to the assignee of the present invention.

Organo-siloxanes are compositions comprising essentially silicon atoms connected to each other by oxygen atoms through silicon-oxygen linkages, thus and organic radicals attached through carbonsilicon linkages to at least some of the silicon atoms. They may be prepared by the hydrolysis of a hydrolyzable organo-mono-silane followed by condensation (partial or complete) of the hydrolysis product. They may also be prepared by hydrolyzing and condensing mixtures of different hydrolyzable organo-monosilanes, as described in the copending application of James Franklin Hyde, Serial Number 432,528, filed February 26, 1942, and assigned to the assignee of the present invention. In the latter case, hydrolyzable silanes which contain no organic radicals attached to silicon through carbon-silicon linkages, such as'silicon tetrachloride or ethyl orthosilicate, may be included with the organo-silanes, if desired. By employing mixtures of silanes, it is possible to prepare o'rgano-siloxanes which contain on the average up to and including three organic radicals per silicon atom.

By hydrolyzable organo-monosilanes, we mean derivatives of SiI-I4, which contain hydrolyzable radicals such as halogens, amino groups, alkoxy, army and acyloxy radicals, etc., and organic radicals that are joined to silicon through carbonsilicon linkage. Examples of such organic radicals are as follows: aliphatic radicals such as methyl, ethyl, propyl, isopropyl, butyl, amyl, hexyl, heptyl to octadecyl and higher; alicyclic radicals such as cyclopentyl, cyclohexyl, etc.; aryl and alkaryl radicals such as phenyl, mono and poly-alkyl phenyls as tolyl, xylyl, mesityl, mono-, di-, and tri-ethyl phenyls, mono-, di-, and tripropyl phenyls, etc., naphthyl, monoand polyalkyl naphthyls as -methylv naphthyl, diethyl na'phthyls, tri-propyl naphthyl, etc.; tetrahydronaphthyl, anthracyl, etc.; aralkyl such as benzyl, phenylethyl, etc.; alkenyl such as methallyl, allyl, etc., and heterocyclic radicals. The above organic radicals may also, if desired, contain inorganic substituents such as halogens, etc.

Hydrolysis of the above silanes or mixtures thereof is generally accompanied by condensation of the intermediately formed hydroxy compounds to form siloxane linkages, thus,

I I -si-o-si The formation of a siloxane linkage generally results from the close approach of two hydroxyl groups andsubsequent elimination of water. It may also result from the close approach of one hydroxyl group to a hydrolyzable group such as halogen, acyloxy, or alkoxy, etc., and subsequent elimination of hydrogen halide, carboxylic acid or alcohol, respectively. Such eliminations are catalyzed by mineral acids, especially hydrochloric and sulphuric, and by alkali metal hydroxides, especially sodium hydroxide. As a result of the hydrolysis and condensation, organosiloxanes are produced Which are partially or completely condensed and which have on the average up to and including three organic radicals attached to each silicon atom. These organo-siloxanes, as previously mentioned, consist essentially of silicon atoms joined together by oxygen atoms through silicon-oxygen linkages and organic radicals attached to silicon through carbon-silicon linkages, the remaining valences, if any, of the silicon atoms being satisfied by hydroxyl radicals and/or by residual unhydrolyzed radicals such as halogens, alkoxy, etc., listed above as the hydrolyzable radicals.

The organo-siloxanes so obtained, some of which are liquids, others solids, differ with respect to their resistance to heat. In general, the partially'condensed siloxanes' undergo a change in physical properties when heated, becoming more viscous, until finally they may become solids. On the other hand, those which are completely condensed, or nearly so, (i. e. substantially free of hydroxyl groups) are extremely resistant to further change due to heat alone. However, even the latter may be further polymerized by contact with acidic agents, alkaline agents, or with air, as disclosed in the copending applications of James Franklin Hyde, Serial Number 481,155, filed March 30, 1943, Serial Number 481,154, filed March 30, 1943; and Serial Number 451,354, filed July 1'7, 1942, all being assigned to the assignee of the present invention.

All the organo-siloxanes, both solid and liquid, undergo a gradual change in properties when exposed to the combined effect of heat and air for a prolonged period of time. This is true even of the completely condensed siloxanes. In the case of the liquid organo-siloxanes, the effect of heat and air is manifested by an increase in viscosity, frequently followed by gelation. This is objectionable where the liquid is being utilized as a hydraulic fluid, dielectric medium and the like. The resinous solid siloxanes, after long exposure to oxygen at elevated temperatures, also undergo a change in properties, becoming less flexible and tough "until eventually they reach an extremely brittle stage. Such changes in properties due to heat or to heat and air combined are ob viousl undesirable.

The primary object of this invention is to stabilize organosiloxanes.

Another object is to provide a method by which changes of properties of organo-siloxanes due to heat and/or oxygen can be prevented.

Another object is to provide a stabilizer for organo-siloxanes.

We have discovered that the stability of an organo-siloxane may be substantially improved by incorporating therein a minor proportion of an aromatic compound having an amino group attached 'to-a carbon atom of the aromatic ring.

From 0.05 to 5 per cent, preferably from 0.1 to 1.5 per cent by weight of the stabilizer may be included in the composition to advantage. Although larger amounts of the stabilizer may be used, if desired, little advantage is gained thereby aswill be shown" below. The so-formed stabilized composition exhibits a marked improvement in resistance to heat and air, and to small quantities of agents which tend to cause polymerization ofthe'siloxanes.

Among the stabilizers of the class described which may be mentioned are p-amino-phenol,

amine reduces its effectiveness as a stabilizer. Accordingly we prefer to use amines which are free of such radicals.

In order to obtain the maximum effectiveness from the aromatic amines, it is necessary to react the latter with the organo-siloxane. This may be accomplished by heating the liquidrsiloxane, to which a small amount of the aromatic amine has been added, to an elevated temperature. The most effective reacting temperature depends, as will be readily appreciated upon the composi tions, both siloxane and amine, involved. Furthermore, the reaction may be efiected at a lower temperature, but the time required for completion is longer. In general, color develops in the solution as the reaction proceeds.

Fora better understanding of the invention, reference should be had to the following examples which'are given merely by way of illustration and are not to be construed as limiting.

EXAMPLE 1 Individual portions of a liquid dimethyl silicone (prepared by the acid catalyzed hydrolysis of dimethyldiethoxysilane) were treated by addition of 1% by weightof various stabilizers after which the different .portions were heated at 230C. The viscosities ofx-the dimethyl silicone before and after heating fOr various lengths of time were determined by measuring the number of seconds required for a given amount of the liquid at room temperature to flow from an arbitrarily chosen capillary pipette. The following table shows the viscosities in seconds after heating for various lengths of time.

TABLE I Viscositz'e's after "heating Hours at 230 C.

0 21 is 43 93 us 329 498 744 .912

p-Aminophenol 19 21.8 p-Hydroxy-N-mono-benzyl aniline 19 21. 5 p-HydroXy-N-N-dibenzyl aniline 19 23. 2 Diphenylamine 19 -2l. 1 No stabilizer 19 53 and NH2CsH4.Si(OC'2H5)3-. In general, we prefer to use those aromatic amines wherein the amino group has a plurality of aromatic rin s attached tothe nitrogen atom since they are outstanding in their stabilizing effect. Furthermore we havediscovered that the presence of sulphur or phosphorouscontaining radicals in the aromatic Itwill be-note'd thatthe sample to which no stabilizer'w'as added became solid "in 48 hours, but samples containing a stabilizer remained liquid for over a month at this elevated temperature.

EXAIVIPLE '2 Liquiddimethyl'silicone wasprepared by refluxing a mixture of dlmethyldiet'hoxysil'ane, ethyl alcohol and aqueous hydrochloric I acid as described in the above mentioned application, Serial No. 432,530. To this liquid which had a viscosity of 1 000' Saybolt seconds -at 30 C. was added a small amount of p-aminophenol. The resulting solution wasiheated at 230C. The viscosity slowlyrose to 6000 Saybolt secondsaIter which there was no "further appreciable change although heating was continued for several WkS.

:3 The value of astabilizer'is strikinglyshown when a relatively volatil stabilizer .is used. A small amount of vdiphen'yl'arnine was addedlto liquid dimethyl silicone and the mixture was heated. As longasthe stabilizer .remameaxas shown "by-its color) the viscosityof the liquid gasses remained low. Twelve hours after the stabilizer had. completely evaporated, the viscosity had risen' sharply, 'and this rapid rise continued to solidification. Y

.A series of copolymers of monoand di-methylsilicon oxides prepared by the hydrolysis of mixceding gelation the smooth curve exhibited a tures .of methyltriethoxysilane and dimethyl-' diethoxysilane in varying molecular proportions were treated with about 1% by weight of p-aminophenol-and were then heated at 230 C. until they gelled. At the sam time corresponding samples without the stabilizer were heated in the same manner. In the following table are listed thehours required to produce gelation for the stabilized and unstabilized methyl silicon copolymers containing different percentages of monomethylsilicon oxide.

TABLE II Hours heated at 230 C. to M] percent of monomethylsilicon produce gelatmn g oxide in copolymer Unstabilized Stabilized 14 Over 507 11 Over 507 26 Over 507 28 Over 507 17 387 10 335 239 4 107 1 59 1 38 .EXANIPLE 5 The efiectiveness of several aromatic amines upon a dimethyl silicone having a viscosity of 488 centistokes at '25" C. was demonstrated by heating the silicone containing about 1% by weight of the amine, at 250 C. In each case, viscosity was plotted against time in hours until the smooth polymerization curve thus obtained made a sharp break upward indicating incipient gelation. In the following table are given in the column under the heading Incipient gelation the hours required to reach the point where the polymerization curve broke sharply upward. In the column under the heading Slope are iven the values obtained by'dividing the total per cent viscosity increase by the time in hours at 250 C. required to reach the point of incipient gelation. These values correspond to the average increase in viscosity per hour.

TABLE HI Inhibitor Imipient e gelation p Nrme 5 55. O p-Amino-phenol 168 4. 5 4-isopropoxy-diphenyl-amine 183 6. 0 Phenyl-2-naphthylamine 64 4. 8 2,4 diamino diphenyl amine. 40 13. 0 4,4 dimethoxy diphenyl amine 150 11. 4 4-hydroxy diphenyl amine 150 13. 4 N ,N diphenyl 1,4 phenylene diamine 150 17. 5 Tetra hydrophenyl 2-naphthylamine 150 24. 2 N,N diphenyl 1,2 ethylene diamine 16 6. 5 N (4-hydroxy phenyl) morpholine. 8 51. 0

EXAMPLE 6 Liquid phenyl ethyl siloxane was prepared by the hydrolysis 'of the mixture of silanes obtained by the reaction of phenyl and ethyl magnesium chlorides on silicon tetrachloride in substantially equimolecular proportions. Its viscosity was 75 centistokes. Two portions of this liquid, one consharp break upward. The slope of the curve up to the sharp break was calculated by dividing the per cent viscosity increase up to the break by the number of hours required to reach that point. In the case of the unstabilized phenyl ethyl siloxane this slope was 43.0 while in the case of the stabilized it was 19.4. The time required to reach the point of incipient gelation (i. e. the point where the polymerization curve broke sharply upward) in the case of the unstabilized phenyl ethyl siloxane was 69 hours while in the case of the stabilized it was 102 hours.

EXAMPLE 7 Liquid phenyl methyl siloxane (prepared by the hydrolysis of the mixture of silanes obtained by the reaction of phenyl and methyl magnesium chlorides on silicon tetrachloride an equimoleoular proportions and having a viscosity of 3320 centistokes) was divided into two parts which were treated exactly as the liquid phenyl ethylsiloxane of Example 6. The slope calculated for the unstabilized phenyl methyl siloxane was 50.0 While for the stabilized it was 14.4. The time required to reach the stage of incipient gelation in the case of the unstabilized p-henyl methyl siloxane was 39 hours while in the case of the stabilized phenyl methyl siloxane it wa 102 hours. i

As pointed out above and as illustrated in the examples, it is essential that the amine be reacted with the liquid organo-siloxane if the maximum efiect is to be obtained. In the above examples, the temperature of testing the stability was in all cases sufficiently high to insure reaction. The following table demonstrates the advisability of reaction taking place between amine and siloxane. A liquid dimethyl silicone was heated with 2% by weight of p-amino-phenol at diiTerent temperature for different lengths of time. Any undissolved p-amino-phenol was then filtered 011. The viscosity changes were plotted against time in hours and the slopes and times required to reach the point of incipient gelation, as defined above, were calculated.

TABLE IV Temp. Incipient Treatment time, hrs. QC gelation Slope It was pointed out above that the concentration of the inhibitor might be as low as .05% by weight and still be effective. In general, the effectiveness of the inhibitor appears to be directly proportional to the concentration of inhibitor which has reacted with the siloxane. Accordingly, it is clear that there is an upper limit of effective inhibitor concentration for a given siloxane. However, under certain conditions of use the siloxane may undergo a change which permits more inhibitor to react with it. In such cases, it may be desirable to have excess inhibitor present or to add more inhibitor periodically to the liquid. The latter method is preferable where the liquid is bein used at suificiently high temperature to cause sublimation or vaporization of oi p-amino-phenol used tostabilize liquiddi-v methyl silicone. The viscosities;v were measured afterintervals of heating at 250,- C. and the. percent viscosity increasecalculated.

TABLE V Per cent. viscosity increase-250- C.

'Oonc. 34hr. 6 hrs. 10 hrs. 28 hrs. 68 hrs.

.0 12.0 Gel 0.1 7. 0. Inc ient' gel. 0. 3: 7. 74. G 0. 6 7. 0 20. 23 l. 0 7. 0 20 31 1.6 7.0 13 28 We have found that thearomatic amines are not only-effective in stabilizing the partiall condensed liquid organo-siloxanes but also the com;

pletely condensed" liquid siloxanessuch as the cycli'cs and the hexa-organo-di-siloxanes. These completely condensed siloxanes are ordinarily quite, stable substances but they undergo oxidation and possibly'rearrangement in the presence of oxygen, at elevated temperatures; whereby the use. of. inhibitors becomes important, However, the'use of'inhibitors is particularly advantageous in the case of liquid organo-siloxanes having on the average from approximately one to approximately two monovalent organic radicals attached to each silicon atom, at least some of the radical's beingalkyl radicals since these siloxanes are particularly sensitive to heat and air at elevated temperatures.

Ingeneral organo-siloxanes treated in accord-. ance withour invention are more resistant to change in physical properties under the influence of heat and air. Specifically the liquid organosiloxanes are thereby rendered more resistantto.

inhibitor; The following-; table shows the. Gigfecttof varyingq-the-amounts impercent by weight,v

WecIaim: 7 I 1 The method of stabilizing an. organo.-.s ilo; r

ane which comprises incorporating tl1erein.sta-.-v

bilizing amounts of a phenyl amine selectedirom the class consisting of phenyl-Z-naphthylamine, 2,4 diamino diphenyl amine, N,N diphenyl 1,4 phenylenediamine, tetrahydrophenyl z -naphthylamine, theorganic substituents of said siloxane consisting essentially of monovalent hydrocarbon radicals attached to silicon through carbon-silicon-linkages;

2. A composition of matter comprising an organo-siloxane and a minor proportion of a phenyl amine, selected from the class consisting of phenyl-2-naphthyl-amine, 2,4 diamino diphenyl? amine, N,N diphenyl 1,4 phenylene diamine, tetrahydrophenyl Z-naphthyI-amine; the organic substituents of said siloxane consisting essentially of monovalent hydrocarbon radicals attached to silicon through carbon-silicon linlgages.

3. A composition of matter comprising a liquid polymeric organo-siloxane and a minor proportion of a phenyl amine selected from the class consisting of phenyl-2-naphthylamine, 2,4 diamino diphenyl amine, N,N diphenyl 1,4 phenylene diamine, tetrahydrophenyl'2-naphthylamine, said organo-siloxane having on the average from approximately one to approximately two mono-- valent hydrocarbon radicals attached to each silicon atom through carbon-silicon linkages, at

least some of said hydrocarbon radicals being alkyl radicals.

4. A composition of matter comprising a liquid organo-siloxane comprising essentially'structur- 211 units of the formula (CH3)2SiO and a minor proportion of a phenyl amine. selected from the class consisting ofphenyl-:2-naphthylamine, 2,4 diamino diphenyl, amine, N,N diphenyl 1,4 pliene ylene diamine, tetrahydrophenyl; 2.-naphthyl a.- mine.

5. A composition of matter comprising aliquld," methyl siloxane comprising essentially structural;

units of the formula (CH3)2SiO and a minor proportion. of phenyl-Z-naphthylamine.

6. A composition'of mattercomprisihg a liquid methyl siloxane comprising essentially structural units of the formula (CH3)2SiO, and a minor proportion of. 2,4 diamino diphenyl amine.

'7. A composition of matter comprising a liquid methyl siloxane comprising essentially structural units of the formula (CHahSiO, and a minor porportion of tetrahydrophenyl 2-naphthylas mine. i

ROB ROY MCGREGOR'; EARL LEATHEN WARRICK. 

